MEMS (Micro Electro Mechanical Systems) which develops since 1960s is known as a revolutionary high-tech in the 21st century. MEMS is a skillful combination of the microelectronic technology and the micro-mechanical technologies, and MEMS is a new multidisciplinary technology developed in recent years that has a revolutionary impact on the future human life.
The fundamental technologies of MEMS include silicon anisotropic etching technology, silicon/silicon bonding technology, surface micro-machining technology, LIGA technology and the like, and these technologies have become essential core technologies for the research and production of MEMS. In silicon-based MEMS processing technology, the most critical processes mainly include anisotropic etching technique of high aspect ratio, bonding technology and surface sacrificial layer technology and the like. The anisotropic etching technology is the key technology of bulk silicon micro-machining, and dry plasma etching technology has become a mainstream of micro-machining technology. With the development of integrated circuit technology, dry etching of silicon trenches with high aspect ratios is no longer a difficult problem. For example, silicon trenches with desired relatively high aspect ratios can be obtained by using inductively coupled plasma, high density plasma etching equipment, etc. However, when the aspect ratios of the silicon trenches are greater than 2 or the depths of the silicon trenches are greater than 3 μm, the deep trenches previously formed will result in the photoresist subsequently formed being trapped in the deep trenches, making it impossible to use conventional spin coating process to obtain a photoresist plane with uniform thickness on a silicon wafer, thereby subsequent photolithographic process can not function normally. As to a sealing cap wafer for bonding of MEMS, it generally has trenches with many more different depths and widths, the depths of the trenches are up to tens or even hundreds of micrometers (μm), therefore the above phenomenon is more serious. Therefore, there is a need to provide a new method for forming silicon trenches with different depths on MEMS sealing cap silicon substrate.